In iOS 4.0 Apple has redesigned the backup process.
iTunes used to store a list of filenames associated with backup files in the Manifest.plist file, but in iOS 4.
I finished my work on this stuff - that is, iOS 4 + iTunes 9.2 update of my backup decoder library for Python - http://www.iki.fi/fingon/iphonebackupdb.py
It does what I need, little documentation, but feel free to copy ideas from there ;-)
(Seems to work fine with my backups at least.)